1. Field of the Invention
The invention relates to techniques for non-destructive dye penetration testing of the state of a surface, such as the dye penetration and magnetoscopic techniques in particular.
Non-destructive dye penetration testing, which is used in particular for checking for the presence of cracks in a surface, entails the application to that surface of a dye under conditions enabling the dye to penetrate into the defects to be detected, illuminating the surface with incident light, and observing the light emitted by the dye present in the cracks and by the residual dye on the surface.
The surface is illuminated by a beam from a mercury vapor lamp, for example, or from a neon tube, and which includes radiation capable of exciting the dye, which responds by emitting visible monochromatic light that can be observed by means of a photosensitive system, for example, possibly associated with means for producing a digital image that is then processed by an image processing system.
The color of the visible light depends on the dye used. It is orange, for example, if the dye is rhodamine 6G.
2. Description of Related Art
A technique of the above kind is described in the publication FR 2 711 426, for example.
The problem arises of optimizing the observation, in particular optimizing the observation of cracks, which is disturbed by the simultaneous observation of an image of residual traces of dye which remain on the surface.
One proposal for optimizing the observation involves processing the digital image, for example as described in the aforementioned publication.
Another problem is determining the depth of the cracks.
The publication FR 2 736 152 describes a method and a dye penetration system for determining the dimensions of defects.
The above two publications indicate the difficulty and the consequential complexity of methods and systems that have been designed to optimize the images and to assess the dimensions of cracks.
The present invention provides a method and a system which are remarkably simple, which optimize the images and which assess the depth of the cracks.
The invention is based on the observation that when the dye is excited by a rectilinear polarized wave the dye in an area of the surface or in a crack emits a rectilinear polarized wave at an angle to the incident rectilinear polarized wave which depends on the thickness of the product in the area or the crack concerned.
The invention uses the angle between the incident rectilinear polarized wave and the rectilinear polarized wave emitted by the dye to eliminate from the observation areas where residual dye is present on the surface and retain only areas in which the dye has penetrated into the cracks. This approach is based on the fact that the angle corresponding to a wave reflected by residual dye on the surface is different from that corresponding to the dye present in a crack because the thickness of the residual dye on the surface is always less than the thickness of dye in a crack.
According to another aspect of the invention, the angles corresponding to the cracks are used to determine the depths of the cracks.
A system for implementing the invention therefore includes means for producing a rectilinear polarized incident wave at a wavelength chosen to excite the dye, transmission means for guiding the wave toward the surface to be studied, observation means for observing rectilinear polarized waves emitted by dye on the surface and in the cracks, and means on the path of the emitted waves, between the surface and the observation means, for selecting emitted waves according to the angle between the incident rectilinear polarized wave and the rectilinear polarized wave emitted by the dye.
The surface under examination is illuminated with a rectilinear polarized wave from a non-polarized light source associated with a polarizer or preferably from a polarized light source.
This applies in particular to a laser, which delivers a rectilinear polarized wave with parallel edges.
The laser has the further advantage of emitting perfectly monochromatic light in a very fine beam which is coherent over great distances.
Rectilinear polarized waves emitted by the dye are selected by one or more polarized wave analyzers on the path of the waves.